Grounded control circuit

ABSTRACT

A material moisture sensing control includes a circuit connectable to a two-wire alternating current supply in either of two polarity postures. The circuit includes an earth grounding of one electrode and is operable for providing the desired consistent sensing and safe operation with either polarity posture while eliminating the need for either an isolation transformer or an auxiliary isolation resistor at the earth ground electrode.

Unlted States Patent 1 [111 3,824,476 Cotton July 16, 1974 GROUNDED CONTROL CIRCUIT 3,521,376 7/1970 Beller 34/45 n entor: u r D. C on, on o a 3,522,660 8/]970 Elders 34/45 [73] Assignee: T Maytag Company Newton Primary ExaminerRudolph V. Rolinec Owa Assistant Examiner-B. P. Davis [22] Filed; Oct. 10, 1973 Attorney, Agent, or FirmWilliam G. Landwier; Rich- 211 Appl. No.: 405,203 Ward Related US. Application Data [63] Continuation of Ser. No. 123,046, March 10, I971, [57] ABSTRACT g i g gggggzg of March A material moisture sensing control includes a circuit connectable to a two-wire alternating current supply [52] U S Cl 328/4 34/45 307/308 in either of two polarity postures. The circuit includes [51] 6 27/00 an earth grounding of one electrode and is operable [58] Fieid /3O8 34/45 for providing the desired consistent sensing and safe 317/5 operation with either polarity posture while eliminating the need for either an isolation transformer or an [56] References Cited 1 auxiliary isolation resistor at the earth ground elect UNITED STATES PATENTS rode 3,497,964 3/1970 Elders 34/45 18 Claims, 9 Drawing Figures ELECTRON 70 FLOW GROUNDED CONTROL CIRCUIT This is a continuation of application Ser. No. l23,046,filed Mar. 10, 1971 which in turn isacontinuation of application Ser. No. 803,637 filed Mar. 3, 1969, now abandoned.

BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to a control system in a machine for treating materials and more particularly to a non-polarity sensitive moisture sensing control for initiating termination of an operation after the materials have reached a predetermined moisture content.

2. Description of the Prior Art One type of control system becoming increasingly used for effecting automatic termination of the drying operation in a fabric drying apparatus includes sensing the electrical conductivity of the fabrics by means of a pair of electrodes in the fabric tumbling container. The prior art patents show a number of specific control circuits for determining fabric dryness by means ofa sensing circuit including a resistance-capacitance circuit portion responsive to the resistance of the fabrics for terminating the dryer operation at a preselected dryness condition. It is essential that charging of the capacitor to a predetermined voltage be consistently achieved under numerous operating conditions. One condition, that of reverse polarity of the circuit, however, is ignored or sidestepped in the prior art patents. If a proper circuit is not provided, a reverse polarity hookup can result in inoperability of the sensing circuit, inconsistent dryness sensing, or even more important, can result in a dead short to the electrodes and create an unsafe electrical conditon of the dryer apparatus.

Most prior art patents show the dryer sensing circuit in association with a 220 volt three-wire system. Many of the prior art sensing systems shown in a three-wire arrangement would result in improper or unsafe operating conditions if the polarity of the circuit were reversed. There is little opportunity, however, that the polarity of the circuit may be reversed because of the nature of a three-wire system and its installation.

Other prior art patents show use of an isolation transformer to avoid having to overcome the polarity 'sensitivity problem.

The polarity sensitivity problem becomes most acute in a drying apparatus designed to operate on 110 volts across a two-wire system. These 110 volt machines include most gas models and the electric models in which the heat input is substantially less than that for the electric model operating on 220 volts. In each of these 1 volt drying devices, it is possible that the machine may be improperly installed or that the two-prong connector may be reversibly inserted into the electrical outlet.

Thus, it becomes clear that there is need for a dryness SUMMARY OF THE INVENTION It is an object of the instant invention to provide an improved material moisture sensing circuit having nonpolarity sensitive operation.

It is a further object of the instant invention to provide an improved material moisture sensing circuit having electrically safe electrodes and being operable to provide uniform moisture sensing in either of two alternate polarity postures.

It is a further object of the instant invention to provide an improved material moisture sensing circuit operable across a power line and a neutral line in either posture of connection and including an earth ground connection with one electrode.

It is a still further object of the instant invention to provide an improved material moisture sensing circuit operable in a safe non-polarity sensitive manner and including an electrode connected at one end into the control circuit through an earth ground path only.

These objects are achieved in a material moisture sensing control reversibly connectable to a two-wire alternating current supply in either of two polarity postures and including a resistance-capacitance circuit portion. One of the pair of electrodes is connected with an earth ground. The circuit for the charging and discharging of the capacitor includes a closed loop path including an earth ground path.

Operation of the device and further objects and advantages thereof will become evident as the description proceeds and from an examination of the accompanying three pages of drawings.

DESCRIPTION OF THE DRAWINGS The drawings illustrate a preferred embodiment of the invention with similar numerals referring to similar parts throughout the several views, wherein:

FIG. 1 is a view of a fabric dryer partially broken away and sectioned and incorporating the control system of the instant invention;

FIG. 2 is an electrical schematic diagram of a preferred circuit embodying the principles of the instant invention;

FIG. 3 is a chart indicating the operation of various switches of the circuit of FIG. 2;

FIG. 4 is a fragmentary portion of a circuit that may be substituted into the circuit of FIG. 2 to provide an alternate circuit embodying the instant invention;

FIG. 5 is a simplified diagrammatic electrical circuit portion showing a specific alternating current half cycle operation of the circuit of FIG. 2;

FIG. 6 is a simplified diagrammatic electrical circuit similar to FIG. 5 and showing an opposite half cycle alternating current operation;

FIG. 7 is a simplified diagrammatic electrical circuit similar to that shown in FIG. 5 with the circuit reversed to show an opposite polarity hookup;

FIG. 8 is a simplified diagrammatic electrical circuit similar to that shown in FIG. 7 and showing the opposite alternating current half cycle operation; and

FIG. 9 is a simplified diagrammatic electrical circuit similar to those shown in FIGS. 5 through 8 but depicting a circuit similar to the polarity sensitive prior art fabric dryness control systems.

DESCRIPTION OF THE PREFERRED EMBODIMENT In FIG. 1 of the accompanying drawings is shown a clothes dryer apparatus having a base 10 that serves as a support for upwardly extending channel members I I and 12 that join with crosspiece 14 to support the hollow blower housing casting 17. The housing 17 includes a tubular portion 21, a divider wall 20 having a rearwardly flared inner portion defining an intake into an impeller chamber, and radially directed longitudinal webs 22 which converge toward a central axis and join with a cylindrical bearing retainer member 23. A plurality of segmental passageways are therefore defined by the tubular member 21, the bearing retainer member 23, and the supporting web 22.

.lournalled within member 23 is a revoluble drum drive shaft 31 projecting from both ends of the housing 17. Fixed to the drum drive shaft 31 adjacent the rear of the dryer is a large pulley 33 which is driven by motor 34 through a motor pulley 36, main drive belt 37, a speed reduction system (not shown) driven by the main drive belt 37, and belt 40 driven in turn by the speed reduction system.

The forward end of the drum drive shaft 31 is rigidly connected to the drum spider member 46 which has radiating spokes 51 that support a rim 52.

A horizontally mounted fabric tumbling drum 60 has a rear wall 61 which is secured to rim 52 for support and rotation by shaft 31. Rear drum wall 61 is imperforate except for a central exhaust opening defined by the circular shoulder 55 that is aligned with the blower tubular member 21. The outer periphery of the rear wall 61 is flanged to form a supporting shoulder for the imperforate cylindrical sidewall 65 which carries the clothes elevating vanes or baffles 66 for tumbling clothing within the drum 60 during rotation thereof. Cylindrical sidewall 65 is connected to the front drum wall The front wall 67 has a centrally located access opening defined by the annular flange 68 and an annular perforate portion 69 located concentrically around the access opening. This perforate portion 69 formed by several concentric rows of holes serves as the air intake to drum 60.

Cabinet 70 is fastened to the base and encloses the entire drying machine. The cabinet has an access opening defined by cabinet flange 71 and aligned with the drum access opening thereby allowing both of the flanges 68 and 71 to receive a rearwardly extending portion of the door gasket 72. The door 73 is hinged Fastened to the cabinet 70 is a shroud or cowling member 74. Located between the shroud 74 and the front drum wall 67 is an open coil electric heating element 75 which extends around the inside of the cowling member 74 to raise the temperature of the air passing through the perforate portion 69 into the drum 60. A gas heater may be used in place of the electrical heating element75.

Airflow into drum 60 through the perforate area 69 and into the blower housing 17 is produced by rotation of the revoluble impeller member 76 located in the blower'housing 17. The fan pulley 77 formed on the rear end of the impeller is driven directly by the driving motor 34 through the main drive belt 37. A felt air sea]- ing member 78 encircles the front periphery of blower housing 17 and the circular shoulder 55 to prevent air from being drawn in at the junction and thus bypassing the heater 75 and drum 60.

The blower housing casting l7 supports at least one thermostat 82 connected in series with the heating element 75 to maintain the interior drum 60 at theproper selected drying temperature. Also connected in series and forms a substantially airtight seal with the gasket with the heating element is the high limit thermostat 79 which is mounted on an upper part of the shroud member 74 so as to disconnect heating element 75 from its source of power in case the temperature near the front of the drum should rise above a predetermined selected temperature during the operation of the clothes dryer, for instance, in the event of reduced airflow through the drum 60.

In order to measure the electrical conductivity or resistance of the fabrics within the drum 60, as a measure of the condition of dryness of the fabrics, electrodes or probes 80 and 81 are mounted within the drum 60. In a preferred embodiment, the electrodes are in the form of a pair of elongated conductor members mounted on the crown of each of the baffle members 66 to provide a contacting surface engageable by the fabrics tumbling within the drum. It will be realized that different forms of electrodes, or probes, may be used, although the type disclosed herein is a preferred construction.

In this embodiment, electrical energy is supplied to electrode 80, for example, by lead 84 that is connected to a brush 85 that engages the stationary slip ring 86 while the drum 60 is rotating. The slip ring 86 is mounted on the electrically insulative band 78. The slip ring 86 is in turn connected to a lead 90 which runs to the control unit 92. The electrode 81 is connected by lead 83 to the rotatable drum 60 and through the drum 60, the spider 46, the drive shaft 31, and blower housing 17 to the chassis including the base 10. The chassis is then connected to earth ground through a conductor 87 electrically connected with the base 10, for example. Electrode 81 is thus connected to earth ground.

The automatic control unit 92 may be securedto an upper portion of the cabinet. The lead 90 enters the control unit and is connected to the control unit to be described hereinafter.

The baffles 66 are formed'of an electrically nonconductive material in order to insulate the electrodes 80, 81 from each other. The electrodes, however, are electrically shorted by the contact of web fabrics during tumbling. A plurality of baffles may be mounted within the drum and each may be provided with electrodes 80 and 81 with the electrode pairs being connected in parallel for achieving a greater sensitivity of control.

Referring now to FIG. 2 which illustrates the control circuitry for the fabric drying apparatus, there is shown diagrammatically a manually operable preselection means including a plurality of switches 101 through 108 actuatable between open and closed positions, according to the chart of P16. 3, under control of push rods or a plurality of manually actuatable push buttons 110 through 113 indicated at Regular, Permanent Press, Damp Dry, and Air Fluff, respectively. The switches'10l through 108 are shown collectively at the right side of the diagram under control of the push-buttons 110 through 113 and are shown schematically in the circuit for controlling initiation and termination of the fabric drying apparatus as will be more fullyshown hereinafter. Operation of the group of switch members 101-108 between open and closed positions is also under control of a selectively energizable solenoid 116, as indicated in FIG. 3, to terminate the drying cycle.

The control circuitry of FIG. 2 includes a pair of conductors 119 and 120 that are selectively connectable with a conventional two-wire single phase alternating current supply through a conventional plug and receptacle arrangement including the plug 121. For the explanation of the circuitry of FIG. 2, it will be assumed that the first conductor 119 is connected with the power line and that the second conductor 120 is connected to the neutral line of the two-wire supply.

The chart of FIG. 3 indicates the electrical posture of the preselection switches 101-108 upon selection of each of the selectable cycles corresponding to the push buttons 110-113 and upon operation of the resetting solenoid 116. It is noted, for example, that the preselection switch 101 in the heater circuit is closed for each of the Regular, Permanent Press, and Damp Dry cycles. In the following explanation of the circuit of FIG. 2, it will be assumed that a Regular cycle has been selected by the operator and thus selection switches 101, 102, 104, 107, and 108 are closed and preselection switches 105 and 106 are open while preselection switch 103 is momentarily closed during the preselection and machine energization process.

The heater 75 is connected between the first and sec ond conductors 119 and 120 by a circuit portion including the preselection switch 101, a cycling thermostat 82, a high limit thermostat 79, and a centrifugal switch 123 in the motor 34. The centrifugal switch 123 is normally open but is operable to a closed position upon energization of the motor 34.

The drive motor 34 is energized by acircuit extending from the first conductor 119 through a door switch 124, closed preselection switch 102, a conductor 125, and momentarily closed preselection switch 103 to one side of the motor 34. The other side of the motor 34 is connected to the second conductor 120. Subsequent to initial energization of the motor 34, centrifugal switch 126 within the motor 34 will operate and a circuit for maintaining operation of the motor will be completed from the first conductor 119 through the door switch 124, preselection switch 102, and conductors 125, 130, 131 to the motor 34.

A cool-down thermostat 133 is also provided in the circuit to the motor 34 and is operable to a closed position at a predetermined temperature within the drum 60 of, for example, 135 F. After the dryer apparatus has operated for a period oftime with heat on, the cooldown thermostat 133 will close and maintain the motor 34 energized until the temperature within the drum 60 is reduced to the 135 F. This cool-down thermostat 133, therefore, provides a fabric cool-down operation following a heat-on drying operation.

The fabric dryness sensing circuit is energized from the first conductor 119, through the door switch 124, selector switch 102, selector switch 107, and conductor 134 to one side of a half-wave rectifier 135. The other side of the rectifier 135 is connected to a pair of parallel related resistors 139 and 140 with a first resistor 139 being in the circuit normally and a second resistor 140 being in parallel with the first resistor 139 upon the closing of selector switch 106 by selection of Damp Dry cycle. The parallel related resistors 139, 140 are then in turn connected to a junction point 141. Extending from the junction point 141 is one circuit portion including a selector switch 104 and a resistor 143 in series with the electrode 80. The other electrode 81 is connected to the drum 60 and through various electrically conductive portions of the apparatus, indicated at 144 in FIG. 2, and to an earth ground through the earth ground conductor 87. Connected to the junction point 141 is a resistor 146 in series with a capacitor 145 that is in turn connected to the second conductor 120.

Also connected to the junction point 141 is one side of a selector switch that is connected at its other side to the second conductor to provide a shunt circuit across the capacitor 145. It will be noted that the capacitor may be charged, as the fabrics across the electrodes 80, 81 become dry, by the dc. circuit extending through the rectifier I35 and resistor 146. The circuit portion including selector switch 105 provides a closed loop path for selectively discharging the capacitor 145 at the end of each cycle. Beginning each cycle with a shorted capacitor insures a consistent relationship of the charge on capacitor 145 to the dryness of the fabrics across the electrodes 80, 81.

A gaseous discharge tube, such as a neom lamp 149, is connected across the capacitor 145. The neon tube 149 normally has an effectively infinite resistance; however, when the charge on the capacitor 145 reaches a predetermined value, the gas within the tube is ionized and a circuit is conducted therethrough. In series with the neon tube 145 is a reed relaycoil 150 responsive to completion of a circuit through the neon tube 145 for closing the juxtaposed reed switch 151. The reed switch 151 completes an energizing circuit to the solenoid 116 for resetting the selector switches 101-108 and thereby initiating termination of the drying operation. The solenoid energizing circuit is completed from the first conductor 119 through the door switch 124, a conductor 153, and selector switch 108 to one side of the solenoid 116. The other side of the solenoid 116 is connected by a conductor 154 to the reed switch 151. The other side of the reed switch 151 is connected to the second conductor 120.

It will be seen from a furtheranalysis of FIGS. 2 and 3 that upon actuation of the solenoid 116 the heating operation will be terminated but thatthe motor 34 will remain energized to effect continued tumbling of the fabrics within the drum 60 for a cool-down operation under operation of the cool-down thermostat 133. Upon the temperature of the air within the drum reaching a predetermined lower temperature, the thermostat 133 will open for de-energizing the motor 34 to terminate the cycle of operation.

Referring to FIG. 4, there is shown an alternate means for actuating the solenoid. The circuit includes a neon tube 149 in series with a current-limiting resistor and responsive to a predetermined voltage on at capacitor as in FIG. 2. Upon the firing of the neon tube 149, a circuit is completed to the gate 156 of an SCR 159 which in turn becomes conductive and completes a circuit between the anode 160 and cathode 161 to effect energization of the solenoid 116. The SCR circuit portion, or the reed relay circuit portion in the case of the circuit of FIG. 2, and the solenoid circuit portion may be considered as an auxiliary circuit responsive to the dryness sensing circuit.

Referring now to FIGS. 5 through 8, the charging of the capacitor during the drying operation will be more specifically discussed.

trical distribution system. It will be noted that FIGS. and 6 are substantially identical except for broken lines 168 and 169 indicating electron flow during each of the two half cycles of the alternating current input. Likewise, FIGS. 7 and 8 are identical except for the broken lines 170 and 171 indicating electron flow during the half cycles of the alternating current input. It is noted that the polarity of FIGS. 7 and 8 is reversed. as compared to FIGS. 5 and 6 and represents the situation in which the plug 21 is turned over and inserted into the receptacle, for example. Such a reverse polarity might also result from improper installation of the dryer or by improper installation of the household wiring. Furthermore, the safety provided by three-prong plugs presently provided with electrical apparatus could be bypassed by removing the grounding prong on the plug or by improperly using an adaptor to permit the operator to unknowingly reverse the plug in the receptacle.

It will be seen that the circuits of FIGS. 5 through 8 include a simplified version of the circuit of FIG. 2 and comprising the rectifier 135, resistors 139, 143, and 146, the electrodes and fabrics across the electrodes as represented by a variable resistor 174, capacitor 145, neon tube 149, the reed relay coil 150, and earth ground conductor 87.

By way of an example of anoperative circuit to achieve the intended drying sensitivity, the resistors 139, 143 and 146 shown in FIGS. 5 through 8 may be of the values 30 megohms, 560,000 ohms, and 100,000 ohms respectively. The capacitor 145 will have a value of approximately 9 microfarads and the neon tube 149 is operable for becoming conductive at approximately 70 volts.

The, connection of the two-prong plug 121 with the two-wire system 163 as shown in FIG. 5 makes the upper conductor 175 the power or hot" line. FIG. 5 is intended to show that part of a full wave in which the upper conductor 175 is positive with respect to the lower conductor 176. Under these conditions, the positive potential at the upper conductor 175 causes electron flow from the earth ground conductor 87 through the electrodes and wet clothes, as shown by the variable'resistor symbol 174, the resistor 143, and through the timing resistor 139 and the rectifier 135 to the upper conductor 175. Electrons also flow from the top plate of the capacitor 145 leaving the top plate positively charged with respect to the bottom plate. This electron flow is shown by the broken line 168 in FIG. 5 and includes a closed loop extending from the broken line as shown, through the upper prong of the connector 121 to the upper wire of the two-wire supply 163, and through the coil 164 to a junction 179. From the junction point 179, one portion of the closed loop extends to earth ground connection 165 and through the earth ground to the earth ground connection 87 at one of the electrodes. The other branch of the closed loop extends from junction 179 through the lower of the two-wire supply, and through the lower prong of the plug 121' and the lower conductor 176 to the lower plate of the capacitor 145. FIG. 5 thus shows the electron flow with a normal polarity connection during the positive half cycle.

The circuit of FIG. 6 also shows normal polarity with the upper conductor 175 being the hot line. The broken line 169 in this figure, however, indicates electron flow during the negative half cycle. Because of the alternating current input, the upper conductor 175 is now negative with respect to the lower conductor 176 and the biasing of the rectifier prevents electron flow from the upper conductor 175. Thus there will be no electron flow through the timing resistor 139 or the rectifier 135. At the beginning of this negative half cycle, the top plate of the capacitor is positively charged with respect to earth ground and therefore electrons will flow from the earth ground connection 87 through the electrodes and fabrics 174 toward the top plate of the capacitor in an attempt to equalize the charge on the capacitor 145. As the fabrics within the dryer become more dry and therefore less conductive, however, the electron flow away from the top plate, as shown in FIG. 5, will exceed the equalizing flow of electrons as shown in FIG. 6. The difference between the flow of electrons from the top plate and the flow of electrons to the top plate will become greater as the fabrics become more dry and the capacitor 145 will gradually build a more positive charge. It is thus seen that at some preselected dryness condition, the capacitor 145 will achieve a predetermined voltage for firing the neon tube 149 connected in parallel across the capacitor.

Referring now to FIG. 7, the circuitry is shown reversed to indicate that the two-prong plug 121 has been reversed to give an opposite polarity to the control circuitry. What was the lower conductor 176 or the neutral line in FIG. Sis now the hot line in FIG. 7. When the conductor 176 is negative with respect to conductor as during the negative half cycle, the rectifier 135 is forwardly biased and electrons fiow away from the bottom of the capacitor 145 through the discharge resistor 146 and then branch through the parallel paths including one path comprising the electrode resistor 143 and the variable resistance 174 of the electrodes and fabrics to the earth ground connection 87. The other electron flow path includes the timing resistor 139 and the rectifier 135. The balance of the closed loop path will include the two-wire supply 163 and earth ground as explained in conjunction with FIG. 5. With this electron flow, the bottom plate of the capacitor 145 will become positively charged with respect to the upper plate as shown in FIG. 7. During the positive half cycle, as shown in FIG. 8, conductor 176 is positive with respect to conductor 175 and the rectifier 135 prevents the flow of electrons through the rectifier 135 and the timing resistor 139. During the positive half cycle, the top plate of the capacitor 145 is now highly positive with respect to ground because of being connected directly to conductor 176 and electrons will flow from the earth ground connection 87 through the electrodes and fabrics 174 and through the electrode resistor 143 and discharge resistor 146 to the bottom plate of the capacitor 145 tending to impart to it a neg- .ative charge.

The negative flow in FIG. 8 is greater in magnitude than the negative flow of FIG. 6 because of the relative electromotive forces causing the flow in each of the circuits. The flow in FIG. 6 is caused by the slightly positive charge on the capacitor 145 as the result of the flow in FIG. 5, whereas the negative fiow of FIG. 8 is the result of the high positive potential on the top plate of the capacitor 145. The increased electron flow' through the parallel paths of FIG. 7 offsets the high opposite electron flow of FIG. 8 and causes both line polarities to permit the control to sense the same moisture retention of the clothes. As the clothes become more dry, the electron flow through the clothes in each of the FIGS. through 8 become smaller and smaller until it is negligible and the timing circuit effects a positive charging of the capacitor 145. Upon the capacitor 145 reaching a predetermined charge, the neon tube 139 is actuated.

It will be seen in FIGS. 7 and 8 that an alternating current flows through the clothes when conductor 176 is the hot side of the line. In this condition, the value of the electrode resistor 143 and the discharge resistor 146 become important from a safety standpoint so that the operator upon touching the electrodes 80, 81 and ground will not be subject to an unsafe current flow. Such a current flow should not exceed 0.2 milliamps.

Referring to FIG. 9, there is shown a simplified electrical diagram similar in part to the diagrams shown in FIGS. 5 through 8. The diagram of FIG. 9, however, shows a prior art circuit portion that is polarity sensitive and thus not operable for achieving satisfactory sensing and safety requirements in both of the two polarity postures. A brief consideration of the polarity sensitive circuit of FIG. 9 will make the advantages of the instant invention more meaningful. In FIG. 9, with the upper conductor 183 connected to the hot line 184, the top plate of the capacitor 185 will become positively charged during the positive half cycle and then will be neutralized during the negative half cycle by the flow of electrons through the resistor 186, the dryer chassis 189, and through the variable resistance 190 of the electrodes and fabrics. As the fabrics become dry, this neutralizing flow of electrons will be decreased and the capacitor 185 will become positively charged. If, however, polarity of the system is changed so that the upper conductor 183 becomes the neutral side of the line, there will be a circuit completed, during the positive half cycle, through the lower conductor 19], which would now be connected to hot line 184, and through the pair of isolation resistors 193, and 186 directly to the earth ground connection 194 through the cabinet or chassis 189 and no charging of the capacitor 185 will result. It is therefore clear that the circuit of FIG. 9, characteristic of prior art devices, is polarity sensitive and thus not as desirable for use in a fabric dryness sensing control circuit subject to a reversed polarity relationship.

It is therefore clear from the above description that the improved non-polarity sensitive fabric dryness sensing circuit as described hereinabove offers distinct advantages for use in a dryer apparatus in an installation where there is a potential reversal of polarity. This is particularly true in two-wire 110 volt installations as versus the 220 volt three-wire installations where proper grounding is more assured. The circuit of the instant invention eliminates the closed loop discharge path through the dryer cabinet and chassis and substitutes a closed loop through earth ground to achieve non-polarity operation without an isolation transformer. Furthermore, the circuit provides consistent sensing of preselected dryness conditions in either polarity posture. Still further, the use of the earth ground path provides a system which is inoperable without an earth ground of the cabinet to assure, in the interest of safety, that the cabinet is properly earth grounded. More specifically, the dryer will shut off after a short period of time, as if no clothes were in the drum, if there is no earth ground connection to the dryer.

In the drawings and specification, there has been set forth a preferred embodiment of the invention and although specific terms are employed these are used in a generic and descriptive sense only and not for purposes of limitation. Changes in form and the proportion of parts as well as the substitution of equivalents are contemplated as circumstances may suggest or render expendient without departing from the spirit or scope of this invention as further defined in the following claims.

I claim:

1. A control for an apparatus and including a control circuit connectable to an electrical supply where one wire of said supply is a power line and another wire is an earth-grounded neutral line, the combination comprising: a first circuit portion; means for connecting said control circuit to said power supply including a first conductor connectable between one side of said first circuit portion and said power line and including a second conductor connectable between the other side of said first circuit portion and said earth-grounded neutral line; means connected in circuit with said first circuit portion and responsive to a predetermined electrical condition of said first circuit portion for initiating a control function; a second circuit portion including a first terminal connected to said first circuit portion and a second terminal effectively electrically disconnected from said first circuit portion and from said second conductor within said control circuit; chassis grounding means forconnecting said second terminal to earth ground through a relatively low resistance chassis path whereby a circuit may be completed from said second terminal to the other side of said first circuit portion through an earth ground path exterior to said control circuit, said second circuit portion being operable 'when incomplete through said chassis path and said earth ground path to said secondconductor for effectively controlling said first circuit portion to produce said predetermined electrical condition whereby initiation of said control function is dependent on the presence of an earth grounding of said apparatus.

2. A control as defined in claim 1 wherein said'first circuit portion includes a capacitor connected between said first and second conductors and wherein said means responsive to a predetermined electrical condition is responsive to a predetermined voltage on said capacitor for interrupting operation of said apparatus and wherein further said second circuit portion is operable as a discharging path for said capacitor to effectively maintain said capacitor discharged and said apparatus operable whereby an earth grounding of said apparatus is essential to operation of said apparatus.

3. A control for an apparatus and includinga control circuit connectable to an electrical power supply where one wire of said supply is a power line and another wire is an earth-grounded neutral line, the combination comprising: means for connecting said control circuit to said power supply including first and second conductors; a first circuit portion effectively connected on one side to said first conductor and on the other side to said second conductor; means connected in circuit to said first circuit portion and responsive to a predetermined electrical condition of said first circuit portion for initiating a control function; and a second circuit portion including a first terminal connected to said first circuit portion and a second terminal effectively electrically disconnected from the other side of said first circuit portion and from said second conductor within said control circuit and connected to a relatively low resistance chassis path to earth ground, said second conductor being disconnected within said control circuit from said low resistance chassis path whereby completion of a circuit through said second circuit portion includes the chassis path and an earth ground path external to said control circuit, said second circuit portion when incomplete being operable for effectively controlling said .first circuit portion to produce said predetermined electrical condition.

4. A control as defined in claim 3 wherein said means responsive to a predetermined electrical condition for initiating a control function comprises means for interrupting operation of said apparatus whereby a proper earth grounding of said apparatus is essential to operation of said apparatus.

5. A control as defined in claim 4 wherein said first circuit porti'on includes a capacitor operable at a predetermined charge for producing said predetermined condition and wherein said second circuit portion is operable for maintaining said capacitor below said predetermined charge when said second circuit portion is connected to' earth ground through said conductive chassis path.

6. A control as defined in claim 4 wherein said second circuit portion includes means for sensing a condition within a'properly grounded apparatus and initiating termination thereof upon'a predetermined condition.

.7. A control for an apparatus and including a control circuit connectable to an electrical power supply where one wire of said supply is a power line and another wire is an earth-grounded neutral line, the combination comprising: .means for connecting said controlcircuit to said power supply including first and second conductors; a first circuit portion including a capacitor connected between said first and second conductors; means responsive to a'predetermined voltage on said capacitor for interrupting operation of said apparatus; and aseries circuit portion including a first terminal connected to one side of said capacitor and a second terminal effectively electrically disconnected from the other side of said capacitor within said control circuit and connected to a relatively low resistance chassis path to earth ground, said second conductor being effectively electrically disconnected from said low resistance chassis path within said control circuit whereby completion of a circuit through said series circuit portion includes the chassis path and an earth ground path external to said control circuit, said series circuit portion being operable as a discharging path for said capacitor to effectively maintain said apparatus operable whereby an earth grounding of said apparatus is essential to operation of said apparatus.

8. A control as defined in claim 7 and further including sensingmeans responsive to a predetermined condition in said apparatus for initiating termination thereof and comprising a pair of electrodes in said series circuit portion normally shunted by low resistance material during operation of said apparatus whereby existence of one of the conditions including the absence of a proper earth ground path and the absence of a low resistance shunt across said electrodes will effect interruption of operation of said apparatus.

9. A material moisture sensing control circuit connectable to an alternating current supply where one wire of said supply is a power line and another wire is an earth grounded neutral line, the combination comprising: means for connecting said moisture sensing control circuit to said alternating current supply including first and second conductors; a resistancecapacitance circuit portion including a capacitor connected between said first and second conductors; trigger means connected in circuit to said capacitor and responsive to a predetermined voltage on said capacitor for initiating termination of operation of a controlled apparatus; electrode means engageable with materials being treated and comprising a first electrode electrically connected to one side of said capacitor and a second electrode spaced from said first electrode and effectively electrically disconnected from the other side of said capacitor and from saidsecond conductor within said control circuit, said materials bridging said electrodes and forming a series circuit portion including said first and second electrodes and said materials bridging therebetween; and grounding means including a conductive chassis grounding path and connecting said second electrode to earth ground, said second conductor being effectively electrically disconnected from said low resistance chassis path within said control circuit and being effectively connected to earth ground for completing a circuit to the other side of said capacitor through said series circuit portion and including said chassis path and an earth ground path exterior to said control circuit, said series circuit portion being operable as a discharging path for said capacitor to effectively maintain said apparatus operable while said materials are wet whereby an earth grounding of said apparatus is essential to operation of said apparatus.

10. A moisture sensing control circuit as defined in claim 9 wherein said pair of conductors are reversibly connectable to said two wires in first and second polarity postures whereby each of said conductors is potentially connectable to the power line and wherein said capacitor is alternately charged and discharged at relative rates dependent upon the moisture content of said materials during successive half cycles of the alternat ing current supply in each of said polarity postures whereby the said control circuit is operable in'either of said polarity postures. 1

ll. A non-polarity sensitive material moisture sensing control circuit controlling an auxiliary circuit and connectable to a single phase alternating current supply where one wire of said supply is a power line and another wire is an earth-grounded neutral line, the combination comprising: means for connecting said moisture sensing control circuit to said alternating current supply including a pair of conductors reversibly connectable to said two wires in first and second polarity postures whereby each of said conductors is potentially connectable to the power line; a rectifier electrically connected to a first of said conductors; a resistance-capacitance circuit portion including a capacitor connected between said first and second conductors in series with said rectifier; trigger means connected in circuit to said capacitor and responsive to a predetermined voltage on said capacitor for initiating actuation of said auxiliary circuit; and electrode means engageable with materials and comprising a first electrode electrically connected to the rectifier side of said capacitor and a second electrode effectively electrically disconnected from the other side of said capacitor within said control circuit and electrically connected through a low resistance chassis grounding path to earth ground, the other of said conductors being effectively electrically disconnected from said chassis grounding path and separately connected to earth ground when said first conductor is connected to said power line, said materials bridging said electrodes and forming a series circuit portion including said first and second electrodes, said materials, said chassis grounding path and said earth ground to said other conductor whereby said series circuit portion is effectively connected in parallel with said capacitor through an earth ground path external to said control circuit, said capacitor being chargeable to said predetermined voltage as said materials assume a predetermined moisture content with said pair of conductors selectively connected to said alternating current supply in said first and second postures.

12. A non-polarity sensitive material moisture sensing circuit as defined in claim 11 wherein said trigger means includes a normally non-conductive discharge device in parallel to said capacitor and operable to a conductive condition at said predetermined voltage for effecting discharge of said capacitor through said discharge device and for actuating said auxiliary circuit.

13. A non-polarity sensitive material moisture sensing circuit as described in claim 11 and including resistance means in series with said capacitor wherein said capacitor is alternately charged and discharged during successive half cycles of the alternating current supply with the discharging of said capacitor being effected through the series circuit portion including the materials and the earth ground path whereby the rate of discharging decreases as the materials acrosssaid electrode means become dry and whereby said capacitor is gradually charged to said predetermined voltage.

14. A non-polarity sensitive material moisture sensing circuit as defined in claim 13 wherein currentlimiting resistance means is connected in the circuit to said first electrode.

15. A non-polarity sensitive material moisture sensing control circuit controlling an auxiliary circuit and connectable to a single phase alternating current supply where one wire of said supply is a power line and another wire is an earth-grounded neutral line, the combination comprising: means for connecting said moisture sensing control circuit to said alternating current supply including a pair of conductors reversibly connectable to said two wires in first and second postures whereby each of said conductors is potentially connectable to the power line with the other conductor being connected to the earth-grounded neutral line; a rectifier electrically connected at a first side to a first of said conductors; and R-C circuit portion including series-connected resistance means and capacitance means, said R-C circuit portion being connected at one side to a second side of said rectifier and at the other side to the second of said conductors; trigger means connected in circuit to said capacitance means and responsive to a predetermined voltage on said capacitance means for actuating said auxiliary. circuit; and electrode means engageable with materials and comprising a first electrode connected in circuit to the rectifier side of said R-C circuit portion and a second electrode effectively electrically disconnected from the other side of said capacitor and said other conductor within said control circuit and electrically connected to earth ground through a low resistance chassis grounding path, said other conductor being effectively electrically disconnected from said chassis grounding path and separately connected to earth ground, said materials bridging said electrodes and forming a series circuit portion including said first electrode, said materials, said second electrode, and said chassis grounding path whereby said series circuit is effectively connected in parallel with said R-C circuit portion only through an earth ground path external to said control circuit, said capacitance means being chargeable to said predetermined voltage as said materials assume a predeter mined moisture content with said pair of conductors selectively connected to said alternating current supply in said first and second postures.

16. A non-polarity sensitive material moisture sensing circuit as defined in claim 15 wherein said trigger means includes a normally non-conductive discharge device in parallel connection to said capacitance means and said series circuit portion and operable to a conductive condition at said predetermined voltage and wherein said capacitance means is alternately charged and discharged during successive half cycles of the alternating current supply with the discharging of said capacitance means being effected through the series circuit portion including the materials and the earth ground path when the charge on said capacitance means is below said predetermined voltage and wherein said capacitance means is at least partially discharged through said discharge device at said predetermined voltage.

17'. A material drying apparatus operable through a drying operation and connectable to a single phase alternating current supply where one wire of said supply is a power line and another wire is an earth-grounded neutral line and having a material dryness sensing control circuit operable for initiating termination of said drying operation, the combination comprising: means for connecting said material drying apparatus to said two wires comprising a pair of conductors reversibly connectable to said two wires in a first posture with a first conductor connected to said power line and in a second reversed posture with a second conductor con-- nected to said power line; a rectifier electrically connected on one side to one of said conductors; an R-C circuit portion including series-connected resistance means and a capacitor, said R-C circuit portion being electrically connected at one side to a second side of said rectifier and at the other side to the other of said conductors;,trigger means connected in circuit to said capacitor and responsive to a predetermined voltage on said capacitor for initiating termination of said drying operation; electrode means including a pair of electrodes engageable with materials for completing an electrical current path therethrough, one of said electrodes being connected through current limiting resistance means to said R-C circuit portion between said rectifier and said capacitor, the other of said electrodes being effectively electrically disconnected from the other side of said R-C circuit portion and from said other conductor within said control circuit; and earth ground conductor means electrically connected to said other electrode and including a low resistance chassis grounding path, said other conductor being effectively electrically disconnected from said earth ground conductor means and separately connected to earth ground, said materials bridging said pair of electrodes gradually charged to said predetermined voltage when said pair of conductors is selectively connected to said alternating current supply in either of said first or second postures.

18. A material drying apparatus as defined in claim 17 wherein said trigger means includes a normally nonconductive discharge device in parallel to said capacitor and operable to a conductive condition at said predetermined voltage on said capacitor. 

1. A control for an apparatus and including a control circuit connectable to an electrical supply where one wire of said supply is a power line and another wire is an earth-grounded neutral line, the combination comprising: a first circuit portion; means for connecting said control circuit to said power supply including a first conductor connectable between one side of said first circuit portion and said power line and including a second conductor connectable between the other side of said first circuit portion and said earth-grounded neutral line; means connected in circuit with said first circuit portion and responsive to a predetermined electrical condition of said first circuit portion for initiating a control function; a second circuit portion including a first terminal connected to said first circuit portion and a second terminal effectively electrically disconnected from said first circuit portion and from said second conductor within said control circuit; chassis grounding means for connecting said second terminal to earth ground through a relatively low resistance chassis path whereby a circuit may be completed from said second terminal to the other side of said first circuit portion through an earth ground path exterior to said control circuit, said second circuit portion being operable when incomplete through said chassis path and said earth ground path to said second conductor for effectively controlling said first circuit portion to produce said predetermined electrical condition whereby initiation of said control function is dependent on the presence of an earth grounding of said apparatus.
 2. A control as defined in claim 1 wherein said first circuit portion includes a capacitor connected between said first and second conductors and wherein said means responsive to a predetermined electrical condition is responsive to a predetermined voltage on said capacitor for interrupting operation of said apparatus and wherein further said second circuit portion is operable as a discharging path for said capacitor to effectively maintain said capacitor discharged and said apparatus operable whereby an earth grounding of said apparatus is essential to operation of said apparatus.
 3. A control for an apparatus and including a control circuit connectable to an electrical power supply where one wire of said supply is a power line and another wire is an earth-grounded neutral line, the combination comprising: means for connecting said control circuit to said power supply including first and second conductors; a first circuit portion effectively connected on one side to said first conductor and on the other side to said second conductor; means connected in circuit to said first circuit portion and responsive to a predetermined electrical condition of said first circuit portion for initiating a control function; and a second circuit portion including a first terminal connected to said first circuit portion and a second terminal effectively electrically disconnected from the other side of said first circuit portion and from said second conductor within said control circuit and connected to a relatively low resistance chassis path to earth ground, said second conductor being disconnected within said control circuit from said low resistance chassis path whereby completion of a circuit through said second circuit portion includes the chassis path and an earth ground path external to said control circUit, said second circuit portion when incomplete being operable for effectively controlling said first circuit portion to produce said predetermined electrical condition.
 4. A control as defined in claim 3 wherein said means responsive to a predetermined electrical condition for initiating a control function comprises means for interrupting operation of said apparatus whereby a proper earth grounding of said apparatus is essential to operation of said apparatus.
 5. A control as defined in claim 4 wherein said first circuit portion includes a capacitor operable at a predetermined charge for producing said predetermined condition and wherein said second circuit portion is operable for maintaining said capacitor below said predetermined charge when said second circuit portion is connected to earth ground through said conductive chassis path.
 6. A control as defined in claim 4 wherein said second circuit portion includes means for sensing a condition within a properly grounded apparatus and initiating termination thereof upon a predetermined condition.
 7. A control for an apparatus and including a control circuit connectable to an electrical power supply where one wire of said supply is a power line and another wire is an earth-grounded neutral line, the combination comprising: means for connecting said control circuit to said power supply including first and second conductors; a first circuit portion including a capacitor connected between said first and second conductors; means responsive to a predetermined voltage on said capacitor for interrupting operation of said apparatus; and a series circuit portion including a first terminal connected to one side of said capacitor and a second terminal effectively electrically disconnected from the other side of said capacitor within said control circuit and connected to a relatively low resistance chassis path to earth ground, said second conductor being effectively electrically disconnected from said low resistance chassis path within said control circuit whereby completion of a circuit through said series circuit portion includes the chassis path and an earth ground path external to said control circuit, said series circuit portion being operable as a discharging path for said capacitor to effectively maintain said apparatus operable whereby an earth grounding of said apparatus is essential to operation of said apparatus.
 8. A control as defined in claim 7 and further including sensing means responsive to a predetermined condition in said apparatus for initiating termination thereof and comprising a pair of electrodes in said series circuit portion normally shunted by low resistance material during operation of said apparatus whereby existence of one of the conditions including the absence of a proper earth ground path and the absence of a low resistance shunt across said electrodes will effect interruption of operation of said apparatus.
 9. A material moisture sensing control circuit connectable to an alternating current supply where one wire of said supply is a power line and another wire is an earth grounded neutral line, the combination comprising: means for connecting said moisture sensing control circuit to said alternating current supply including first and second conductors; a resistance-capacitance circuit portion including a capacitor connected between said first and second conductors; trigger means connected in circuit to said capacitor and responsive to a predetermined voltage on said capacitor for initiating termination of operation of a controlled apparatus; electrode means engageable with materials being treated and comprising a first electrode electrically connected to one side of said capacitor and a second electrode spaced from said first electrode and effectively electrically disconnected from the other side of said capacitor and from said second conductor within said control circuit, said materials bridging said electrodes and forming a series circuit portion including said first and second electrodes And said materials bridging therebetween; and grounding means including a conductive chassis grounding path and connecting said second electrode to earth ground, said second conductor being effectively electrically disconnected from said low resistance chassis path within said control circuit and being effectively connected to earth ground for completing a circuit to the other side of said capacitor through said series circuit portion and including said chassis path and an earth ground path exterior to said control circuit, said series circuit portion being operable as a discharging path for said capacitor to effectively maintain said apparatus operable while said materials are wet whereby an earth grounding of said apparatus is essential to operation of said apparatus.
 10. A moisture sensing control circuit as defined in claim 9 wherein said pair of conductors are reversibly connectable to said two wires in first and second polarity postures whereby each of said conductors is potentially connectable to the power line and wherein said capacitor is alternately charged and discharged at relative rates dependent upon the moisture content of said materials during successive half cycles of the alternating current supply in each of said polarity postures whereby the said control circuit is operable in either of said polarity postures.
 11. A non-polarity sensitive material moisture sensing control circuit controlling an auxiliary circuit and connectable to a single phase alternating current supply where one wire of said supply is a power line and another wire is an earth-grounded neutral line, the combination comprising: means for connecting said moisture sensing control circuit to said alternating current supply including a pair of conductors reversibly connectable to said two wires in first and second polarity postures whereby each of said conductors is potentially connectable to the power line; a rectifier electrically connected to a first of said conductors; a resistance-capacitance circuit portion including a capacitor connected between said first and second conductors in series with said rectifier; trigger means connected in circuit to said capacitor and responsive to a predetermined voltage on said capacitor for initiating actuation of said auxiliary circuit; and electrode means engageable with materials and comprising a first electrode electrically connected to the rectifier side of said capacitor and a second electrode effectively electrically disconnected from the other side of said capacitor within said control circuit and electrically connected through a low resistance chassis grounding path to earth ground, the other of said conductors being effectively electrically disconnected from said chassis grounding path and separately connected to earth ground when said first conductor is connected to said power line, said materials bridging said electrodes and forming a series circuit portion including said first and second electrodes, said materials, said chassis grounding path and said earth ground to said other conductor whereby said series circuit portion is effectively connected in parallel with said capacitor through an earth ground path external to said control circuit, said capacitor being chargeable to said predetermined voltage as said materials assume a predetermined moisture content with said pair of conductors selectively connected to said alternating current supply in said first and second postures.
 12. A non-polarity sensitive material moisture sensing circuit as defined in claim 11 wherein said trigger means includes a normally non-conductive discharge device in parallel to said capacitor and operable to a conductive condition at said predetermined voltage for effecting discharge of said capacitor through said discharge device and for actuating said auxiliary circuit.
 13. A non-polarity sensitive material moisture sensing circuit as described in claim 11 and including resistance means in series with said capacitor wherein said capacitor is alternately charGed and discharged during successive half cycles of the alternating current supply with the discharging of said capacitor being effected through the series circuit portion including the materials and the earth ground path whereby the rate of discharging decreases as the materials across said electrode means become dry and whereby said capacitor is gradually charged to said predetermined voltage.
 14. A non-polarity sensitive material moisture sensing circuit as defined in claim 13 wherein current-limiting resistance means is connected in the circuit to said first electrode.
 15. A non-polarity sensitive material moisture sensing control circuit controlling an auxiliary circuit and connectable to a single phase alternating current supply where one wire of said supply is a power line and another wire is an earth-grounded neutral line, the combination comprising: means for connecting said moisture sensing control circuit to said alternating current supply including a pair of conductors reversibly connectable to said two wires in first and second postures whereby each of said conductors is potentially connectable to the power line with the other conductor being connected to the earth-grounded neutral line; a rectifier electrically connected at a first side to a first of said conductors; and R-C circuit portion including series-connected resistance means and capacitance means, said R-C circuit portion being connected at one side to a second side of said rectifier and at the other side to the second of said conductors; trigger means connected in circuit to said capacitance means and responsive to a predetermined voltage on said capacitance means for actuating said auxiliary circuit; and electrode means engageable with materials and comprising a first electrode connected in circuit to the rectifier side of said R-C circuit portion and a second electrode effectively electrically disconnected from the other side of said capacitor and said other conductor within said control circuit and electrically connected to earth ground through a low resistance chassis grounding path, said other conductor being effectively electrically disconnected from said chassis grounding path and separately connected to earth ground, said materials bridging said electrodes and forming a series circuit portion including said first electrode, said materials, said second electrode, and said chassis grounding path whereby said series circuit is effectively connected in parallel with said R-C circuit portion only through an earth ground path external to said control circuit, said capacitance means being chargeable to said predetermined voltage as said materials assume a predetermined moisture content with said pair of conductors selectively connected to said alternating current supply in said first and second postures.
 16. A non-polarity sensitive material moisture sensing circuit as defined in claim 15 wherein said trigger means includes a normally non-conductive discharge device in parallel connection to said capacitance means and said series circuit portion and operable to a conductive condition at said predetermined voltage and wherein said capacitance means is alternately charged and discharged during successive half cycles of the alternating current supply with the discharging of said capacitance means being effected through the series circuit portion including the materials and the earth ground path when the charge on said capacitance means is below said predetermined voltage and wherein said capacitance means is at least partially discharged through said discharge device at said predetermined voltage.
 17. A material drying apparatus operable through a drying operation and connectable to a single phase alternating current supply where one wire of said supply is a power line and another wire is an earth-grounded neutral line and having a material dryness sensing control circuit operable for initiating termination of said drying operation, the combination comprising: means for connecting said mateRial drying apparatus to said two wires comprising a pair of conductors reversibly connectable to said two wires in a first posture with a first conductor connected to said power line and in a second reversed posture with a second conductor connected to said power line; a rectifier electrically connected on one side to one of said conductors; an R-C circuit portion including series-connected resistance means and a capacitor, said R-C circuit portion being electrically connected at one side to a second side of said rectifier and at the other side to the other of said conductors; trigger means connected in circuit to said capacitor and responsive to a predetermined voltage on said capacitor for initiating termination of said drying operation; electrode means including a pair of electrodes engageable with materials for completing an electrical current path therethrough, one of said electrodes being connected through current limiting resistance means to said R-C circuit portion between said rectifier and said capacitor, the other of said electrodes being effectively electrically disconnected from the other side of said R-C circuit portion and from said other conductor within said control circuit; and earth ground conductor means electrically connected to said other electrode and including a low resistance chassis grounding path, said other conductor being effectively electrically disconnected from said earth ground conductor means and separately connected to earth ground, said materials bridging said pair of electrodes and forming a series circuit portion including said pair of electrodes, said materials, and said earth ground conductor whereby said series circuit is effectively in parallel with said capacitor only through an earth ground path external to said control circuit, said capacitor being alternately charged and discharged during successive half cycles of the alternating current supply with the discharging of said capacitor being effected through the series circuit portion including the materials whereby the rate of discharging decreases as the materials become dry and whereby said capacitor is gradually charged to said predetermined voltage when said pair of conductors is selectively connected to said alternating current supply in either of said first or second postures.
 18. A material drying apparatus as defined in claim 17 wherein said trigger means includes a normally non-conductive discharge device in parallel to said capacitor and operable to a conductive condition at said predetermined voltage on said capacitor. 